Planetary Nebulae are fascinating objects, out of the ordinary from collections of stars, and excellent beginner targets. M27, the Dumbbell Nebula, is probably the best, as it is large, easy to find, and has an interesting shape and structure.

Warning:It looks far better in dark skies, so save your first look for a dark sky site if that is at all possible.

Planetary Nebulae are shells of gas ejected by stars in the late stages of their life. When an ordinary mass star (such as our own Sun) expends all its Hydrogen fuel, internal fusion stops. This stops the outward radiation pressure, which causes the gas to begin to collapse under its own gravity. The collapse raises the internal pressure and temperature enough to start a new fusion reaction, with the Helium gas that was the byproduct of the earlier Hydrogen fusion. The radiation from this fusion expands the star, which lowers the pressure, which stops the fusion. The gas then collapses, and fusion restarts, and so on.

This on-off cycling is called Thermal Pulsing and, each time the direction of collapse or expansion reverses, an outer layer of gas is “puffed off” into space. Successive puffs build up a structured shell of gas at some distance from the star, and this gas glows in the radiation of the inner stellar core. When we look at this shell of gas from a distance, we tend to see the edges more than the near face, since at the edges we are looking through more gas; this is why we tend to see a shape with a distinct outline (often a circle) and not a globe of gas.